“Someone said my system needs balancing. What does that mean?”
The idea
A system produces a total amount of conditioned air. Balancing decides how that total gets divided between rooms, on purpose, based on what each room needs rather than on where the ducts happened to run.
It is done with measurements at each register and adjustments at dampers and takeoffs, then re-measured until the division matches the design.
What it is not
It is not walking the house opening and closing registers until things feel better. Without knowing what each room needs and what each register is delivering, that is just moving the problem around — and, as above, it can raise system pressure and reduce total airflow.
When balancing is the right answer
Balancing solves an uneven division of adequate air. It does not manufacture air that the system is not producing.
If total airflow is already short — high static pressure, an undersized return, a restrictive filter — balancing will just redistribute the shortage. That is why pressure and total airflow get measured first, and correction of the restriction comes before balancing.